stirred tank bioreactors - ferdowsi university of...

BIODESULPHURISATION

STIRRED TANK BIOREACTORS

Presented by: Zohre Salmani

Superviser: Dr.R.Gheshlaghi

Date: May 2012

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OUTLINE:

1. Introduction :Biodesulfurization 2. Standard geometry of a stirred tank bioreactor 3. Headspace volume 4. Basic features of a stirred tank bioreactor 4.1. Agitation system 4.1.1 Top entry and bottom entry impellers 4.1.2 Mechanical seals 4.2 Oxygen delivery system 4.2.1 Compressor 4.2.2 Air sterilization system 4.2.3 Positive pressure 4.2.4 Sparger 4.2.5 Effect of impeller speed

4.3 Foam control

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INTRODUCTION

Sulfur is the most undesirable element present in

petroleum .

Combustion of fossil fuels leads to the atmospheric

emission of sulfur oxides, which are a major cause of acid

rain .

Corrosion in transferring pipes

Adverse effects on health ,environment and economy

Sulfur content in Our country Disel: 5000 to 7000 ppm , but

the standard sulfur content should be 5 to 50 ppm.

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Conventional desulphurization

HDS Oxidative Adsorptive Biodesulfurizatin

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There is a HDS plant in Tabriz and Shahriar , 72000

barrel in a day

DESULPHURIZATION PROCESSES

HDS(Hydrogen Desulphurization)

Sulfur is catalytically converted to hydrogen sulfide in the

presence of hydrogen

• Oxidative desulphurization

Organic components which contain sulfur in an

oxidation reaction change to their solfones.

• adsorptive desulphurization:

• *Physical : sulfur is adsorbed on a solid

adsorbent

• * Chemical :at first they remove sulfur in a form

of hydrogen sulfide then they remove hydrogen

sulfide with the help of an adsorbent 5

DRAWBACKS

• High pressure(150-200 psig) and high temperature(200-450 c)

• High operating cost

• Not capable to remove DBT

• Release of carbon dioxide

HDS

• Not all kinds of sulfur content composition can be able to participate in an oxidation reaction

Oxidative

• High pressure(25 bar), high temperature (400 c) Adsorptive

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BIODESULFURIZATION

Removing sulfur using biotechnological processes with the

help of microorganisms.

Whole microorganisms as well as their enzymes can use a

wide range of compounds as substrates transferring them

into specific products.

Microorganisms:

Rhodococcus erythropolis(IGTS8)

Xanthomonas

Pseudomonas putida

T.thiooxidans

T.acidophilus

This process occurs at ambient temperature and pressure

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BIOREACTORS IN BIODESULPHURIZATION

Electro- spray bioreactors

Trickle- bed bioreactor

Stirred Tank Bioreactor

….

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Trickle bed: another variation of the packed bed, fluid is sprayed onto the

top of the packing and trickles down through the bed. Air is introduced at

the base, because liquid is not continuous throughout the column, so air

moves easily around the packing.

STIRRED TANK BIOREACTOR The most important bioreactor for industrial application is

the conventional mixing vessel, which has the advantages

of low operating investment .

Vessels for laboratory experiments of volume up to 20 liters

are made of glass.

For larger vessels ,construction is made of stainless steel

they are constructed according to recognized standards

such as those published by the International Standards

Organization and the British Standards Institution.

These dimensions take into account both mixing

effectiveness and structural considerations

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STIRRED TANK BIOREACTOR

A mechanically stirred tank bioreactor

fitted with

a sparger and

a Rushton turbine

will typically have the following relative

dimensions:

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GEOMETRY OF BIOREACTOR

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DIMENSIONS

Height: Diameter ratio of the vessel can vary

between 2:1 and 6:1, depending largely on the

amount of the heat to be removed and the

stirrers may be top or bottom driven.

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BIOREACTOR

• A bioreactor is divided in a working volume and a head-space volume.

• The working volume is the fraction of the total volume taken up by the medium, microbes, and gas bubbles.

• The remaining volume is called the headspace.

• Typically, the working volume will be 70-80% of the total fermenter volume.

• This value will however depend on the rate of foam formation during the reactor. If the medium or the fermentation has a tendency to foam, then a larger headspace and smaller working volume will need to be used.

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BAFFLES

All tanks are fitted with baffles ( you know why?)

4 baffles are used for vessels less than 3m

diameter and 6-8 baffles are used for larger

vessels

T/12 < width of the baffle < T/10 (T is the tank

diameter)

10< Dr / Db < 12 ,(T=tank , b=baffle)

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BASIC FEATURES OF A STIRRED TANK

BIOREACTOR

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DIFFERENT PARTS OF A STIRRED TANK

BIOREACTOR

* An agitator system

* An oxygen delivery system

*A foam control system

* A temperature control system

* A pH control system

* Sampling ports

* A cleaning and sterilization system.

* A sump and dump line for emptying of the reactor

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AGITATION SYSTEM

The function of the agitation system is to

provide good mixing and thus increase

mass transfer rates through the bulk

liquid and bubble boundary layers.

provide the appropriate shear conditions

required for the breaking up of bubbles.

The agitation system consists of the agitator and

the baffles.

The agitator consists of the components shown in

the following diagram:

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AGITATION SYSTEM

The agitator consists of the components shown in the

following diagram:

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AGITATION

The number of impellers will depend on the

height of the liquid in the reactor. Each impeller

will have between 2 and 6 blades. Most microbial

fermentations use a Rushton turbine impeller.

Speed control or speed reduction devices are used

to control the agitation speed.

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TOP ENTRY AND BOTTOM ENTRY

IMPELLERS

The impeller shaft can enter from the bottom of the tank or

from the top. A top entry impeller ("overhung shaft") is

more expensive to install as the motor and the shaft will

need to be structurally supported:

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BOTTOM ENTRY

A reactor with bottom entry impeller however will need

higher maintenance due to damage of the seal by

particulates in the medium and by medium components

that crystallize in the seal when reactor is not in use:

Bottom entry agitators tend to require more maintenance

than top entry impellers due to the formation of crystals

and other solids in the seals

Dimensionless numbers

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MECHANICAL SEALS

The mechanical seal is used to prevent

contaminants from entering the reactor and to

prevent organisms from escaping through the

shaft.

The seal uses vapors from the liquid for

lubrication.

It is therefore important that you do not turn the

shaft when the tank is dry so as not to damage

the seal.

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OXYGEN DELIVERY SYSTEM

The oxygen delivery system consists of

a compressor

inlet air sterilization system

an air sparger

exit air sterilization system

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COMPRESSOR

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COMPRESSOR

A compressor forces the air into the reactor. The compressor will need to generate sufficient pressure to force the air through the filter, sparger holes and into the liquid.

Air compressors used for large scale bioreactors

typically produce air at 250 kPa. The air should

be dry and oil free so as to not block the inlet air filter or contaminate the medium.

Note that it is very important that an "instrument air" compressor is not used. Instrument air is typically generated at higher pressures but is aspirated with oil. Instrument air compressors are used for pneumatic control

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OXYGEN TRANSFER RATE ,(KL A)

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C=Constant: depends on the geometrical parameters of the vessel

and stirrer employed

Vs =superficial gas velocity

µa = liquid effective viscosity

P/V = The average power input per volume

Table 2

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OXYGEN MASSTRANSFER

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Where Kl a is the volumetric mass transfer coefficient in s-1;

α is proportionality factor, as a constant;

Pg is the agitator power under gassing conditions in W;

Vl is the liquid volume without gassing in m3;

Vg is the gas superficial velocity in m/s;

and y and z are empirical constants.

The mass transfer coefficient for none-Newtonian filamentous

fermentation :

AIR STERILIZATION

Sterilization of the inlet air is undertaken to prevent contaminating organisms from entering the reactor.

The exit air on the other hand is sterilized not only to keep contaminants from entering but also to prevent organisms in the reactor from contaminating the air.

A common method of sterilizing the inlet and exit air is filtration. For small reactors (with volumes less than 5 liters), disk shaped hydrophobic Teflon membranes housed in a polypropylene housing is used. are used. Teflon is tough, reusable and does not readily block. 32

FILTERATION

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For larger laboratory scale fermenters (up to 1000 litres), pleated

membrane filters housed in polypropylene cartridges are used.

FILTRATION

By pleating the membrane, it is possible to create a compact

filter with a very large surface area for air filtration.

Increasing the filtration area decreases the pressure

required to pass a given volume of air through the filter.

Sterilization of the inlet and exit air in large bioreactors (>

10,000 liters) can present a major design problem. Large scale

membrane filtration is a very expensive process. The filters

are expensive as they are difficult to make and the energy

required to pass air through a filter can be quite considerable.

Heat sterilization is alternative option. Steam can be used to

sterilize the air. With older style compressors, it was possible

to use the heat generated by the air compression process to

sterilize the air. However, compressors are now multi-stage

devices which are cooled at each stage and disinfecting

temperatures are never reached. 34

CONDENSER

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In small reactors, the exit air system will typically include a condenser.

CONDENSER

The condenser is a simple heat exchanger

through which cool water is passed

Do you know what is it for?

Volatile materials and water vapor condense on the inner condenser surface.

This minimizes water evaporation and the

loss of volatiles.

Drying the air also prevents blocking of

the exit air filter with water

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AIR STERILIZATION SYSTEM -

POSITIVE PRESSURE

During sterilization the concept of "maintaining

positive pressure" will often be used.

Maintaining positive pressure means that during

sterilization, cooling and filling and if appropriate, the

fermentation process, air must be pumped into the reactor.

In this way the reactor is always pressurized and thus

aerial contaminants will not be "sucked" into the reactor.

It is very important that positive pressure is maintained

when the bioreactor is cooled following sterilization.

Without air being continuously pumped into the reactor, a

vacuum will form and contaminants will tend to be drawn

into the reactor.

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AIR STERILIZATION SYSTEM -

POSITIVE PRESSURE

Maintaining positive pressure at all stages of the

fermentation setup and operation is an important aspect of

reducing the risk of contamination

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Without aeration, a

vacuum

forms as the reactor cools.

With aeration, positive

pressure is always

maintained and

contaminants are pushed

away from the reactor

SPARGER

The air sparger is used to break the incoming air

into small bubbles.

Although various designs can be used such as

porous materials made of glass or metal, the

most common type of filter used in modern

bioreactors is the sparge ring:

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SPARGER

A sparger ring consists of a hollow tube in which small holes have been drilled. A sparger ring is easier to clean than porous materials and is less likely to block during a fermentation.

The sparger ring must located below the agitator and will have approximately the same diameter as the impeller.

Thus, the bubbles rise directly into the impeller blades, facilitating bubble break up. 40

EFFECT OF IMPELLER SPEED

the shear forces that an impeller generates play a

major role in determining bubble size. If the

impeller speed is to slow then the bubbles will

not be broken down. In addition, if the impeller

speed is too slow, then the bubbles will tend to

rise directly to the surface due to their buoyancy.

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IMPELLER SPEED

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Slow impeller speed

The bubbles will not be

sheared into smaller bubbles

and will tend to rise directly

towards the surface

Fast impeller speed

Smaller bubbles will be generated

and these bubbles will move with

throughout the reactor increasing

the gas hold up and bubble residence

time

IMPELLER SPEED

Another consequence of too slow an impeller speed is a flooded impeller.

Under these conditions, the bubbles will accumulate and coalesce under the impeller, leading to the formation of large bubbles and poor oxygen transfer rates.

A similar phenomenon will happen when aeration rate is too high.

In this case, the oxygen transfer efficiency will be low 43

- FOAM CONTROL SYSTEM

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Foam control is an essential element of the operation of a sparged

bioreactor. The following photograph shows the accumulation of foam

in a 2 litre laboratory reactor.

FOAM CONTROL SYSTEM

Excessive foam formation can lead to blocked air exit filters and to pressure build up in the reactor.

The latter can lead to a loss of medium, damage to the reactor and even injury to operating personnel.

Foam is typically controlled with aid of antifoaming agents based on silicone or on vegetable oils.

Excessive antifoam addition can however result in poor oxygen transfer rates. 45

The antifoam requirement will depend on

the nature of the medium.

Media rich in proteins will tend to foam more readily than

simple media.

the products produced by the fermentation.

Secreted proteins or nucleic acids released as a result of

cell death and hydrolysis have detergent like properties.

the aeration rate and stirrer speed.

Increasing the aeration rate and stirrer speed increases

foaming problems.

the use of mechanical foam control devices

Foam control devices such as mechanical and ultrasonic

foam breakers help to reduce the antifoam requirement.

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FOAM CONTROL

The head space volume

The larger headspace volume, then the greater the

tendency for the foam to collapse under its own weight. For

example, for fermentations in which high levels of foam is

produced, a 50% headspace volume may be required.

Condenser temperature

In laboratory scale reactors, a cold condenser temperature

can help to control the foam. The density of the foam

increases when it moves from the warm headspace volume

to the cold condenser region. This causes the foam to

collapse.

Some antifoam are toxic to the cells.

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FOAM IS TYPICALLY DETECTED USING TWO

CONDUCTIVITY OR "LEVEL" PROBES.

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When the upper level probe is above the foam

level, no current will pass between the level

probes and the antifoam pump remains

turned off.

When the upper level probe is

immersed in the foam layer, a current is

carried in the foam. This causes the

antifoam to turn on.

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TWO STAGE BDS UNIT

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PROCESS

In the first stage the model diesel is contacted

with the resting cells. After 24hr of reaction , the

organic and cell-containing aqueous phases are

separated by centrifugation. Fresh cells are then

added to the recovered organic phase for a

further 24 h period.

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SUMMARY

Biodesulfurization

Aware of standard geometry of a stirred

tank bioreactor

Know the basic features of a stirred tank

bioreactor

Understand working of the agitation system

Components of the oxygen delivery system

Foam control

Multistage desulfurization

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EXAMPLE

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Wish you a happy life ,

Thanks for listening